Try compiling with -source 1.4 -target 1.4 and see if it works then. This should remove the references to StringBuilder. Or you could try upgrading your GCJ first and see if the latest version includes StringBuilder.

I tried compiling same code with -source 1.4 -target 1.4, it works. I don't have any idea about GCJ, so I didn't tried. Please elaborate GCJ.

Make sure you are using Sun Java instead of GCJ. You're using GCJ instead of Sun Java 6, despite Java 6 being installed on your machine. How you can make sure that you're using Java 6 instead of GCJ is something that you could best ask on a forum about Red Hat Linux.